HTTP protocol implementation: allow to configure which protocol version(s) to use

[sebastian-nagel]

It should be possible to configure which HTTP protocol versions (HTTP/1.1, HTTP/2, etc.) are used by a protocol implementation:

• WARC readers may fail on yet unknown HTTP protocol versions (see discussion "Is there an option to use HTTP/1.0 or 1.1 for the Warc File", Do not use "http/2" protocol version in HTTP headers in WARC files commoncrawl/news-crawl#42)
• but could be also useful for debugging and testing

[jnioche]

fixed in #829